1. Field of the Invention
The present invention relates to output devices incorporated in semiconductor integrated circuit devices and responsive to an input signal for supplying a signal of high level or low level.
2. Description of the Background Art
There is a need for semiconductor integrated circuit devices capable of driving a large load at high speed because of the recent requirement for speeding-up of the operation of digital devices. In semiconductor integrated circuit devices, a large number of circuits are highly integrated on a small area of a substrate and multiple interconnection layers are formed on the substrate. Therefore, if the recent semiconductor integrated circuit devices are operated at high speed and with a large current, the interconnecting lines tend to form a so-called distributed constant circuit.
Accordingly, there is a problem that a reflected wave as well as a microwave circuit should be taken into consideration. This problem will now be described with reference to FIGS. 14 to 17.
FIG. 14 is a conventional output device in a C-MOS integrated circuit device. Referring to FIG. 14, the output device includes a P channel transistor 1P, an N channel transistor 1N, an input terminal 11 commonly connected to gate electrodes of P channel transistor and N channel transistor 1N and an output terminal 12 commonly connected to drain electrodes of P channel transistor 1P and N channel transistor 1N. P channel transistor 1P has its source electrode coupled to a power supply V.sub.DD and N channel transistor 1N has its source electrode coupled to a ground potential. A capacitive load 14 is connected to output terminal 12 by an interconnecting line 15. Interconnecting line 15 includes an interconnecting line or a bonding wire formed on a substrate.
In operation, P channel transistor 1P and N channel transistor 1N are complementarily turned on/off in response to an input signal. Accordingly, the output device consumes a very small amount of power.
FIG. 15 is an equivalent circuit diagram of the output device shown in FIG. 14. In the equivalent circuit diagram shown in FIG. 15, N channel transistor 1N is in an ON state, supplying a signal of "L" level to output terminal 12. Referring to FIG. 15, N channel transistor 1N includes a current source 16 and an on resistance Ron. An interconnecting line 15 connected to a load 14 includes an inductance L. Load 14 has a capacitance C.
Accordingly, "L" level output circuit constitutes a resonance circuit including on resistance Ron, inductance L and capacitance C. The resonance circuit has the following characteristic.
Suppose that a resonance frequency f0 and a quality factor in the resonance frequency is Q, then ##EQU1## wherein .omega..sub.o =2.pi.f0.
It is necessary to improve the current handling capability (namely conductance) in order to speed up the operation of the output device shown in FIG. 14. One way to improve the current handling capability is to make the channel width larger than the channel length of the gate. FIG. 16 shows an output voltage-output current characteristic with the current handling capability being thus improved. Referring to FIG. 16, (1) represents a characteristic before the current handling capability is improved, and (a) represents an on resistance value near "L" level in that case. (2) represents a characteristic with the current handling capability being improved and (b) represents an on resistance value near "L" level in that case. As is obvious from FIG. 16, the on resistance (output voltage/output current) is decreased if the current handling capability is improved. The on resistance Ron near the expected logical value ("L" level in this case) is also lowered. As can be seen from the equation (2) above, when the on resistance Ron is decreased, the value of Q becomes high and the resonance circuit is activated by the current at the time of the falling of the output signal and causes vibration. This happens when it supplies "H" level as well as "L" level. Ringing is caused at output terminal 12 by such a vibration. FIG. 17 shows an output voltage waveform at output terminal 12 at this time.
If the load is a transmission line, mismatching of the impedance tends to occur, causing ringing by reflection. The ringing presents noises in transmitting signals, resulting in generation of interference waves, i.e., spurious radiation as well as malfunction of a logic circuit system. The interference waves could cause a malfunction of other electronic appliances.
As stated above, the conventional output device presents a problem of ringing caused when the current handling capability of the output transistor is enhanced in order to speed up the operation. One way to restrain the ringing is to connect a damping resistor in series with output terminal 12. However, this is not preferable for speeding up the operation as it attenuates the output current.
Another possible way (through rate control) is to control the activation level of the resonance circuit by providing a specific value of di/dt for the rising/falling of an input signal supplied to the gate of the output transistor to reduce the change in the current of the input signal. However, this approach is not preferable in speeding up the operation since it provides a specific di/dt for the input signal.